Disharmony works even if the attacker was not tapped to attack.

The creature untaps (if it was tapped) before you get control of it.

Disharmony can target a creature you control.

The current wording explicitly removes the creature from combat. It is no longer an attacking or blocking creature.

If you cast Disharmony during an opponent’s turn, you may then block with the creature you gain control of (assuming there’s an attacking creature it can block).

If a turn has multiple combat phases, this spell can be cast during any of them as long as it’s before the beginning of that phase’s Declare Blockers Step.